The present invention relates to a system for indicating the position of moving elements. More particularly, the present invention relates to an improved well apparatus for indicating the position of a first element movable relative to a second element constructed from a magnetically permeable material. The invention is especially applicable to signal communication between the interior and exterior surfaces of a pressure vessel positioned downhole in a well.
Downhole well tools are positioned in hydrocarbon wells to perform different functions. Well tools such as sliding sleeves, safety valves, and packers include tool components that move relative to the other elements. Such well tools are attached to cylindrical pipe and typically incorporate pressure sensitive electronic components, guages, sensors, pyrotechnic devices, and other components requiring protection from high well pressures. Because the well pressures can exceed 30,000 psi, the pressure vessels and communication port seals must withstand such pressures without permitting fluid leakage past the fluid seals. The integrity of such seals deteriorate over time and can cause failure of the pressure sensitive well tool components. Additionally, space limitations within a downhole well tool restricts the size and configuration of signal communication devices engaged with the pressure sensitive well tool components.
Various switches have been developed to detect the relative position of moving components. For example, U.S. Pat. No. 4,355,280 to Duzich (1982) disclosed a traveling magnet attached to a spring in an accumulator. A Hall Effect sensor sensed the travel of the magnet, and "flux gathering material" was positioned to increase the sensitivity of the Hall Effect sensor. In U.S. Pat. No. 5,197,508 to Gottling et al. (1993), a magnet was positioned on a movable valve member, and a field sensor detected the position of the magnet. The field sensor was fastened in a magnetically nonconductive housing. In U.S. Pat. No. 5,414,355 to Davidson et al. (1995), a magnet was positioned relative to a carrier, and a Hall effect sensor was positioned to detect movement of the magnet.
None of these devices communicate signals through the wall of a downhole well tool body. In a well, the cylindrical pipe of a well tool body is formed with a carbon steel alloy, a martensitic stainless steel, or other magnetically permeable material capable of withstanding the well fluid pressures. These magnetically nonpermeable materials cooperate with the enclosed tool body configuration to create an effect known as the Faraday Shield effect. Faraday Shields are used to isolate electronic components from strong magnetic fields because a magnetic field will not pass radially through a hollow cylinder constructed from a magnetically permeable material. Consequently, the flux from a traveling magnet in a well pipe cannot be reliably detected by a magnetic sensor located outside of the magnetically permeable well pipe.
The communication of signals through a downhole pressure vessel wall in a well tool has been accomplished in two ways. Magnetic sensors have been positioned within the pressure vessels to detect a moving magnet and to generate an electrical signal, and the sensor signal is transmitted by a sealed feedthrough device through the pressure vessel wall. One feedthrough device is constructed with an electrically nonconductive ceramic material sealed by an O-ring seal to an aperture through the pressure vessel. An electrically conductive pin through the ceramic material communicates electric signals through the pressure vessel wall. However, such feedthrough devices are undesirable because they create potential leak paths, are limited to electric signals, and utilize pin elements which extend outwardly beyond the pressure vessel wall. This latter feature can interfere with other well components in the confined space of the well tool.
To avoid the leakage problems of feedthrough devices, magnetic sensors have been attached to the outer surface of a magnetically nonpermeable pressure vessel to detect the position of a magnet attached to a moving element within the pressure vessel. However, such nonpermeable pressure vessels are constructed from high alloy steels such as Inconel which significantly increase the overall cost of the downhole well tools.
Accordingly, a need exists for an improved sensor capable of detecting relative movement between components of a well tool, and of transmitting a signal representating such movement. The sensor should be operable in cooperation with a tool body constructed from a magnetically permeable material.